The so-called MFB technique has been known, according to which the acceleration of the diaphragm of a speaker is detected by a certain method and the detected output is negatively fed back to a power amplifier thereby to lower the lowest resonance frequency of the speaker.
In the case when a microphone is used a method for detecting the acceleration of the diaphragm of a speaker, an angle arm is provided in front of the diaphragm. The microphone is fixed in the center part of the angle arm and the sound pressure reproduced by the diaphragm is detected by the microphone. This method has the advantage of detecting vibration without any contact between the diaphragm and the microphone. However, according to the above method, the angle arm for fixing the microphone which protrudes in front of the diaphragm disturbs the radiation of sound; the angle arm resonates; the external appearance is undesirable; and furthermore, since the speaker and the microphone should be always built in as a pair, there is a problem that the freedom of design is limited.
Especially, after MFB is applied, the low frequency region is enhanced and the displacement of the diaphragm increases. In order to avoid any touch of the diaphragm with the angle arm, the angle arm should be kept at a distance from the diaphragm. However, if this is done, there arises a problem that the feedback upper-limit frequency (the maximum frequency at which the amount of feedback becomes 0 dB) becomes low since the rotation of the phase of sound waves from the diaphragm to the microphone increases. Moreover, if the microphone is provided in front of the speaker, the influence of the so-called Doppler distortion due to the Doppler is unavoidable.